Nonaqueous-electrolyte secondary batteries including lithium secondary batteries are being put to practical use in extensive applications ranging from power sources for applications for so-called public use, such as portable telephones and notebook type personal computers, to vehicle-mounted power sources for driving motor vehicles or the like and stationary large power sources or the like. However, recent nonaqueous-electrolyte secondary batteries are increasingly required to have higher performance, and are required to attain battery characteristics, such as, for example, high capacity, high output, high-temperature storability, and cycle characteristics, on a high level.
Especially in the case where lithium secondary batteries are for use as power sources for electric vehicles, the lithium secondary batteries are required to have high output characteristics and input characteristics because electric vehicles necessitate a large amount of energy when started and accelerated and because the energy which generates in a large amount upon deceleration must be efficiently regenerated. Furthermore, since electric vehicles are used outdoors, the lithium secondary batteries are required to have high input/output characteristics (have low internal impedance) especially at a low temperature such as −30° C. in order that the electric vehicles can be smoothly started and accelerated even in the cold season. In addition, the lithium secondary batteries must deteriorate little in capacity and increase little in internal impedance even when repeatedly charged and discharged in a high-temperature environment.
Meanwhile, when lithium secondary batteries are used not only in electric-vehicle applications but also as stationary large power sources, such as various backup applications, applications for leveling the load of electric power supply, and applications for stabilizing the output of electric-power generation by natural energy, then not only cells having an increased size are used but also a large number of cells are connected serially or in parallel. Because of this, problems concerning reliability and safety due to various kinds of non-uniformity, such as unevenness in discharge characteristics among the individual cells, unevenness in temperature among the individual cells, and unevenness in capacity or charged state among the individual cells, are apt to arise. In case where a cell assembly such as that described above is improperly designed or regulated, this poses a problem, for example, that only some of the cells constituting the cell assembly are kept in a highly charged state or that the internal temperature of the battery rises, resulting in a high-temperature state.
Namely, the current nonaqueous-electrolyte secondary batteries are required to attain the following items on an exceedingly high level: to have a high initial capacity and high input/output characteristics, to have a low internal impedance, to have a high capacity retention after a durability test, such as a high-temperature storage test or a cycle test, and to be excellent in terms of input/output performance and impedance characteristics even after the durability test.
Many techniques have hitherto been investigated with respect to various battery components including positive-electrode and negative-electrode active materials and nonaqueous electrolytic solutions, as means for improving the input/output characteristics, impedance characteristics, high-temperature cycle characteristics, and high-temperature storability of nonaqueous-electrolyte secondary batteries. For example, patent document 1 describes that when LiFSO3 is used as an electrolyte, a battery which has a high discharge capacity when evaluated for 60° C. charge/discharge cycle characteristics is obtained. Patent document 1 includes a statement to the effect that when LiClO4 is used as an electrolyte, the LiClO4 decomposes because of the noble potential of the positive-electrode active material to generate active oxygen and this active oxygen acts on the solvent to accelerate a solvent decomposition reaction. The document further includes a statement to the effect that when CF3SO3Li, LiBF4, and LiPF6 are used as electrolytes, decomposition of the electrolytes proceeds because of the noble potential of the positive-electrode active material to generate fluorine and this fluorine acts on the solvent to accelerate a solvent decomposition reaction.